The mouse Fc receptor which binds mouse IgGl and IgG2a immune complexes will be purified using a recently isolated monoclonal antimouse Fc receptor antibody to construct an affinity column. Milligram quantities of the antigen will be isolated to facilitate biochemical analysis of the antigen. A second set of monoclonal antibodies obtained after immunization with the purified antigen will be used to analyze domains of the receptor and to facilitate the analysis of the biosynthesis and dynamics of turnover of the receptor.